1. Technical Field
The present invention relates in general to enhanced efficiency of data access in a data processing system and in particular to a method and system for automated transducer head switching in a data storage subsystem within a data processing system. Still more particularly, the present invention relates to a method and system which permit automated transducer head switching in any selected order in a data storage subsystem.
2. Description of the Related Art
Data processing systems frequently include large scale storage devices such as direct access storage devices (DASD) which are located externally to the host computer system and sometimes located at significant distances therefrom. Communication from the host computer system to the DASD is accomplished over signal cables referred to as data channels which extend between the DASD and its control unit and connect the DASD units to the host computer system.
Current technology provides DASD units with several separate disks, all rotatable about the same spindle. These disks or platters are then accessed by head disk assemblies with a transducing head providing access to one surface of each disk. There may be, for example, nine platters in a disk drive providing sixteen usable surfaces with one of the usable surfaces used for maintaining accurate tracking capability. In such units there are thus fifteen usable surfaces for data and when all heads are positioned, a cylinder of fifteen physical, recording tracks may be accessed.
DASD units frequently use a so-called "Count-Key-Data" architecture (CKD) wherein records which are written on a track are provided with a count field (an identification), a key field and a data field. In writing these fields along a recording track, a gap is provided between each of the fields. These gaps are sometimes utilized to provide a time period during which the DASD control unit and the host computer system data channel may communicate with each other. It is during this gap time that the storage system control unit provides information back to the channel in response to the command which has been received and obtains the next command in order to begin the next operation for searching, retrieving or writing records. In a synchronous system the particular record upon which the DASD unit is working is the same record on which the channel has requested work, so that both the data channel and the DASD unit are synchronous with each other in the sense that they are both working on the same record, either to read that record or to write it.
As data processing systems become faster and faster, the delays created by gaps or by the performance of functions within a gap period must be minimized to the extent that these functions can be performed within such gaps. This is particularly true of optical fiber channels, such as the ESCON (or Enterprise System Connection) fiber optic channel manufactured by International Business Machines Corporation of Armonk, N.Y.
One manner in which these functions may be eliminated to enhance the efficiency of data storage subsystems is the so-called "roll" mode of operation in which processing begins with the first record encountered on a particular recording track. Thereafter, when the end of the track is encountered, the operations "rolls" to the beginning of the recording track and continues. In this manner, all records on a particular track may be accessed within a tracked cyclic storage device without requiring the transmission of control signals between the storage subsystem controller and the direct access storage device.
A natural extension of this technique is a roll mode operation over multiple tracks. U.S. patent application Ser. No. 07/575,741, filed Aug. 31, 1990, entitled "Non-Synchronous DASD Control," now U.S. Pat. No. 5,157,770 which is assigned to the assignee herein, describes a system which permits channel programs to describe the nature and scope of a data transfer before the first data transfer command is executed. This is accomplished utilizing a bitmap which includes a plurality of bits, each bit corresponding to a particular track within a tracked cyclic storage device. By altering the value of the bit at each place within the bitmap, a plurality of recording tracks may be accessed by sequentially enabling only those heads associated with the selected recording tracks. However, this technique, while a substantial advance over the prior art, requires the translation of this bitmap into the relevant control parameter for selecting a particular transducer head and the subsequent transmission of that control parameter to the tracked cyclic storage device. Additionally, the utilization of a sequential series of digital bits to control the switching of transducer heads requires that the transducer heads be selected in either ascending or descending numerical order, without permitting variation of that order.
In a modern data processing storage subsystem, the accessing of multiple recording tracks within a tracked cyclic storage device by switching the transducer heads must also be accomplished as rapidly as possible in order to minimize the amount of overhead associated with these control signals. Further, those skilled in the art will appreciate that it would be desirable to implement a data storage subsystem which permits multiple tracks within a tracked cyclic storage device to be automatically accessed in any desired order.